TOXICITY OF ACIDS TO INFUSORIA 69 



and in conjunction either with their sodium salts or with HCl. 

 The technique is substantially the same as that reported in my 

 first paper. The only differences are that the organisms were 

 washed in twice-distilled water instead of in pond-water, and that 

 the test-tubes were of Pyrex instead of ordinary soft glass. Ordi- 

 nary distilled water is very toxic, but the toxicity very largely 

 disappears after a second distillation over glass from acid solu- 

 tion (Lyon). The time at which two-thirds of the organisms 

 ceased to move their cilia was taken as the average time of death. 

 The length of life observed in any solution would of course be 

 the reciprocal of the toxicity. 



To 100 cc. portions of the organic acid solution one or more 

 drops of 0.1 N HCl were added and the pH of each portion was 

 determined by means of indicators. The toxicity of the mix- 

 tures was then tested and compared with HCl of the same pH and 

 with the organic acid alone. In the preliminary experiments the 

 procedure was to test the different acids on successive days, 

 first without HCl, next day with one drop of HCl, then with 

 two drops, and so on. This is a fairly satisfactory method if 

 the results are checked by frequent repetition at considerable 

 intervals. Further study has shown that Euplotes is apt to 

 vary in resistance most unexpectedly, especially in late winter; 

 consequently it is a better procedure to test all of the mixtures 

 simultaneously and so under precisely the same conditions. 



In the preliminary experiments ('19) the addition of one or 

 two drops of HCl seemed to lessen the toxicity to Euplotes of 

 several of the acids. This result, however, is not obtained if 

 the day-to-day variation is eliminated by testing the mixture 

 simultaneously. The results are given in table 1. The addition 

 of one or even two drops- of HCl, although it increases the pH of 

 the mixture somewhat and must force the already very slightly 

 dissociated organic acid almost wholly into molecular form, 

 leaves the toxicity of the mixture almost unchanged. A further 

 addition of HCl produces^ marked increase in toxicity which then 

 follows closely the curve for HCl alone. The fact that toxicity 

 is not increased by the first addition of HCl indicates that such 

 a mixture is a trifle less toxic as compared with HCl than is 



THE JOURNAL OF EXPERIMENTAL ZOOLOGT, VOL. 34, NO. 1 



